1. Field of the Invention
The present invention relates to a manufacturing method for a single crystal of calcium fluoride with a large diameter (.o slashed. 200 mm or greater) and superior optical properties, which can be used appropriately in an optical system as the lens and windows of various devices using KrF, ArF excimer lasers and F.sub.2 lasers (such as a stepper, CVD device, or nuclear fusion device), and in particular, for photolithographic devices with a wavelength of 250 nm or less (such as photolithographic devices that utilize KrF, ArF excimer lasers and F.sub.2 lasers), and a single crystal of calcium fluoride for photolithography (a wavelength of 200 nm or less).
2. Discussion of the Related Art
Currently, VLSI has been experiencing increasingly higher integration and higher functionalization. In the field of theoretical VLSI progress is being made on a "system on a chip" in which larger systems are loaded on a chip. Along with this, microscopic processes and higher integration have been required for wafers, for example of silicon, which is the substrate. In photolithography, in which microscopic patterns of integrated circuits are exposed and transcribed on a wafer, for example of silicon, an exposing device called a stepper has been used.
Using DRAM as an example of VLSI, recently, a capacity of 256 M or more has been realized and the width of the processed line has become very small (0.35 .mu.m or less). Therefore, the projection lens of the stepper, which is the key to photolithographic technology, requires superior image formation performance (resolution, focal depth).
The resolution and focal depth are determined by the wavelength of the light used for the exposure and the numerical aperture (NA) of the lens. When the exposure wavelength .lambda. is the same, smaller patterns have a larger angle of diffracting light, and therefore, unless the NA of the lens is large, the diffracting light cannot enter. In addition, the shorter the exposing wavelength .lambda., the smaller the angle of the diffracting light, and therefore, a small NA for the lens is acceptable.
The resolution and the focal depth are expressed in the following formulas:
Resolution=k1*.lambda./NA, and PA1 Focal depth=k2*.lambda./(NA).sup.2 PA1 where k1 and k2 are proportional constants.
From the formulas above, it is understood that in order to improve the resolution, it is desirable to increase the NA of the lens (to enlarge the diameter of the lens) or to shorten the exposure wavelength .lambda.. Shortening .lambda. is more advantageous from the point of view of the focal depth.
First, the shortening of the wavelength of light is described. The exposure wavelength .lambda. has continued to become shorter, and steppers that use KrF excimer laser light (wavelength of 248 nm) as the light source have appeared on the market. There are very few optical materials that can be used for photolithography in the short wavelength range of 250 nm or less. There have been two kinds of materials, calcium fluoride and silica glass that have been used.
Next, the enlargement of the diameter of the lens is described. Simply having a large diameter is not enough. A silica glass or a single crystal of calcium fluoride with superior optical properties, such as uniformity of the refractive index, is required. Here, an example of a manufacturing method for a single crystal of calcium fluoride from the prior art is described. A single crystal of calcium fluoride has been manufactured through the Bridgeman method (Stockburger method, crucible lowering method). In the case of a single crystal of calcium fluoride used in the range of ultraviolet or vacuum ultraviolet light, natural calcium fluoride is not used as the material. In general, a high-purity material produced through chemical synthesis is used. Although it is possible to use powdered material, in this case, there is significant mass reduction during fusing, and therefore, normally, half-fused products or the crushed form are used.
First, a crucible filled with said material is placed in a growth device. Then, the inside of the growth device is maintained with a vacuumed atmosphere of 10.sup.-3 to 10.sup.-4 Pa. Next, the temperature inside the growth device is raised to the melting point of the calcium fluoride or higher, and the material inside the crucible is fused. At this point, in order to prevent temporal changes of the temperature inside the growth device, the temperature is controlled by a constant electrical output or a high-precision PID control.
At the stage of crystal growth, gradual crystallization occurs from the bottom of the crucible by lowering the crucible at a speed of 0.1 to 5 mm/h. The crystal growth is finished at the point when it is crystalized to the top portion of the fused liquid. Then, it undergoes simple and gradual cooling, avoiding sudden cooling, so that the grown crystal (ingot) will not be cracked. When the temperature inside the growth device is reduced to room temperature, the device is released to the normal atmosphere and the ingot is removed.
In the case of calcium fluoride for small-sized optical parts and windows where uniformity is not required, after the ingot is cut out, it is processed to the final product via a process such as rounding. In the case of a single crystal of calcium fluoride used in, for example, the projection lens of a stepper, which requires high uniformity, simple annealing is applied to the ingot. Then, after it is cut and processed to the appropriate size for the product with a given purpose, further annealing is applied.
In Patent Publication Tokukai Heisei 8 (1996)-5801, there is a disclosure of calcium fluoride for photolithography in which the optical path difference due to double refraction in either direction of the 3 axes is 10 nm/cm, when it is used in the specific wavelength band of 350 nm or less. The influence that the optical path difference imparts to the imaging performance of the optical system is expressed as a numerical value which is the multiple of the wavelength (such as .lambda./10), and the smaller the coefficient, the smaller the influence.
For example, with an optical path difference of 10 nm, when the wavelength .lambda.=248 nm, the optical path difference becomes 10/248=0.040.lambda., and when .lambda.=193 nm, the optical path difference becomes 10/193=0.052.lambda.. In other words, even with the same optical path difference of 10 nm, the result is that a .lambda. of 193 nm has a greater effect and the imaging performance becomes worse. Therefore, for the projection lens of a stepper that be used with the next generation ArF excimer laser (wavelength of 193 nm), an optical path difference of 10 nm/cm is not adequate enough. Thus, calcium fluoride with an even smaller optical path difference due to double refraction is required.
Hereinafter, the optical path difference per unit length due to double refraction is simply called double refraction. Also, in general, this double refraction is often called strain. This is because even if there is no double refraction in the material itself, double refraction is often generated due to strains.
As described above, calcium fluoride is manufactured through the Bridgeman method. After calcium fluoride is grown through the Bridgeman method, the calcium fluoride is gradually cooled at a rate at which it will not crack (or to the extent that cutting is possible) and then it is taken out as an ingot. Sometimes, an ingot is directly cut to size for a given purpose, however, the greater the mass, the greater the double refraction and the nonuniformity of the refraction, index. Therefore, said ingot is cut into multiple blocks and then goes through a further annealing process so that the quality is improved.
From the point of view of productivity, in general, the period of this annealing process has, in the past, been one to two weeks. Therefore, the cooling speed during the cooling stage, which takes a large percentage of the time of the entire annealing process, has been set to be 10 to 5.degree. C./hour. However, a single crystal of calcium fluoride obtained through this type of annealing (thermal process) for a single crystal of calcium fluoride has problems in that the refractive index has poor uniformity and the double refraction is too large. Therefore, it is difficult to obtain a single crystal of calcium fluoride that can be used in an optical system for photolithography. In particular, the problem existed in which a single crystal of calcium fluoride with a large diameter (.o slashed. 200 mm or greater) that could be used for photolithography with at wavelength of 250 nm or less could not be obtained.
Accordingly, the present invention is directed to a manufacturing method for calcium fluoride and calcium fluoride that substantially obviates one or more of the problems due to the limitations and disadvantages of the related art.